Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages that are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
Rapidly emerging measurement-while-drilling (MWD) technology introduces a new, deep (3-10 meters) scale for an electromagnetic logging application related to well navigation in thick reservoirs. The major problem associated with the MWD environment is the introduction of a metal drill pipe close to the area being measured. This pipe produces a very strong response and significantly reduces the sensitivity of the measured EM field to the effects of formation resistivities and remote boundaries. Previous solutions for this problem typically comprise creating a large spacing (up to 20 meters) between transmitter and receiver. However, the sensitivity of such a tool to remote boundaries is low.
U.S. Pat. No. 7,150,316 to Itskovich, having the same assignee as the present invention and the contents of which are incorporated herein by reference, teaches an apparatus for use in a borehole in an earth formation and a method of using the apparatus. A tubular portion of the apparatus includes a damping portion for interrupting a flow of eddy currents. A transmitter positioned within the damping portion propagates a first transient electromagnetic signal in the earth formation. A receiver positioned within the damping portion axially separated from the transmitter receives a second transient electromagnetic signal indicative of resistivity properties of the earth formation. A processor determines from the first and second transient electromagnetic signals a resistivity of the earth formation. The damping portion includes at least one cut that may be longitudinal or azimuthal. A non-conductive material may be disposed within the cut. Alternatively, the damping portion may include segments having cuts and segments having a non-conducting material on an outer surface thereof.
It has been found that the device of Itskovich provides the ability to determine a distance to an interface in the earth formation in which the borehole is inclined at angles of less than 45 degrees to the interface. The term “interface” is intended to include a boundary between two fluids in an earth formation and also a boundary between different layers of the earth formation. At larger inclinations, the resistivity sensor may be considered to be “looking ahead of the drill” and the ability to identify interfaces 10 m ahead of the bottomhole assembly is relatively poor. These larger angles are commonly encountered when first drilling into a reservoir.
U.S. Pat. App. Pub. No. US 2007/0216416 A1 to Itskovich, having the same assignee as the present invention and the contents of which are incorporated herein by reference, discloses an apparatus for evaluating an earth formation. The apparatus includes a downhole assembly conveyed in a borehole in the earth formation. The downhole assembly may include a member having a finite, non-zero conductivity. A transmitter associated with the downhole assembly produces a first transient electromagnetic signal in the earth formation. A receiver receives a second transient electromagnetic signal resulting from interaction of the first transient electromagnetic signal with the earth formation, the receiver being spaced apart from the transmitter. An electromagnetic shield associated with the downhole assembly reduces an effect on the second transient electromagnetic signal of substantially direct coupling between the transmitter and the receiver. A magnetostatic shield associated with the downhole assembly reduces an effect on the second transient electromagnetic signal of currents induced in the downhole assembly by the first transient electromagnetic signal. The electromagnetic shield may comprise a highly conductive material.
There is a need to design an induction system suitable for both geo-steering and measurements ahead of the drill bit. The system should be capable of looking up to five (5) meters ahead of the drill bit and up to ten (10) meters deep for the task of geo-steering. These requirements imply that, in the case of a vertical well, the system should be capable of seeing the approaching resistivity bed boundary up to five (5) meters in advance before the drill bit crosses the boundary. Similarly, in the case of geo-steering in a horizontal well, the system should be capable to see the approaching resistivity water-oil boundary located up to ten (10) meters way from the well. The present disclosure describes how to fulfill that need.